In the production of oil and gas from the earth, drilling and completion fluids are commonly recirculated through a filter. The benefits of clean completion and drilling fluids have been well established. The most common method of filtering the completion and drill-in fluids has been the use of diatomaceous earth (sometimes briefly “DE”) filters and cartridge filters. In either case, the conventional filters are satisfactory for removing simple contamination, but frequent shutdowns are required to remove cake from the DE filters and to replace the cartridge filters. Cake or other solids buildup is detected or assumed from an increase in pressure or a decrease in flow rate. Cake and/or other solids cannot normally be removed by backflow or otherwise from cartridge filters, which utilize porous media. The expensive cartridge filters must be replaced.
Unfortunately, caking and solids loading of the filters are not the only cause of decreased flow or increased pressure. The widespread use of gelling agents, viscosifying agents and the like in brines, drilling mud, cleaning sweeps, and other well fluids greatly increases the incidence of filter fouling. They may be introduced to the fluid in the form of dissolved powder, circulation “pills,” viscosifying solutions, and by any other means or in various solutions known to the hydrocarbon production art. The gels or viscous liquids can include, most commonly, hydroxyethylcellulose (HEC), but xanthan gum, various guar gum derivatives, polyacrylamide and other synthetic water soluble polyacrylates are frequently introduced to wellbores.
When circulated gel-inducing agents reach a fluid return tank or holding tank, they can remain intact or become mixed into the completion fluids. Almost immediately as the gel-inducing agents enter a filter, they are likely to blind off a DE filter or plug a cartridge filters. This blinding off requires a total shut down and cleaning of the filter as previously described. In the case of the DE filter, a minimum of one hour is required before filtration can resume, costing valuable rig time and expense. Replacing cartridge filters is likewise time consuming and expensive as some filter pods may hold as many as fifty (50) elements per housing.
In the current practice of filtration of well treatment fluids, there is no defense against the sudden introduction of fouling polymers and gels in the fluid entering the filters. Sudden and drastic reductions in flow and increases in pressure are common and the operators of the filters must be prepared almost without notice to shut down the filters and take action to return them to working order. Such interruptions in normal procedures are very expensive, especially in off-shore rigs and other remote sites.
Another difficulty in the re-use of well treatment fluids, and the preparation of fresh ones, is that polymeric additives can be present in a concentration too high or too low. In the case of recycled fluid, where several desired additives are present also but in varying concentrations too dilute for effective use, the question of how most efficiently to adjust them while also adjusting the polymer concentration is perplexing.